In the manufacture of integrated circuits and related technology, organic composition are commonly used as stripping solutions for photoresists. Both positive and negative resists are stripped using mixtures drawn from a group of substances including phenolics, sulfonic acids, chlorocarbons and hydrocarbons. Several commercially available stripping solutions are constituted in this way. Chief among disadvantages for some of these products is the toxic and skin penetrating nature of some components making handling personally hazardous and waste disposal by conventional dumping and landfill procedures undesirable. The inclusion of hydroscopic sulfonic acid components tends to cause water absorption that leads to corrosivity toward semiconductor substrate metals. Reaction between aluminum metal and chlorocarbon can yield low levels of chloride on substrate surfaces which in turn translates into device damage through corrosion.
When photolithography device manufacture involves only positive photoresists, stripping may be done with a number of organic mixtures comprised of any of a number of polar solvents and mixtures of solvents. An indication of this is described by W. S. DeForest (Photoresist: Materials and Processes, McGraw-Hill, Inc. 1975). Some of these stripping compositions include dimethylformamide, N-methyl-2-pyrrolidone diethanolamine, triethanolamine, several glycol ethers (e.g., such as those available commercially under the brand names Cellosolv and Carbitol), and ketones (i.e. methylethylketone, acetone, methylisobutylketone, cyclohexanone). A number of stripping solutions currently on the market utilize mixtures of some of these organic solvents and are designed for use exclusively in removing positive resists.
The difficulty of photoresist stripping depends on process conditions used in device manufacture which may be determined by the type of device made. When most positive resists are cured by baking no higher than 150.degree. C., removal using most solvent based stripping solutions is achieved at relatively low temperatures near normal room temperature. With bake temperatures between 150.degree. and 200.degree. C. photoresist stripping is more difficult and may require using higher temperatures for the stripping process. Where photoresist films or patterns are exposed to plasma and ion implantation conditions during dry processing, very hard to strip material is formed.
From experience with available stripping solutions, it has been found that mixtures comprised of "neutral" polar solvents such as amides (DMF, N-methyl-2-pyrrolidone, etc.) glycol ethers (Cellosolv, Carbitols, etc.) sulfones (e.g., sulfolanes), ketones (e.g., acetone, methylethylketones and the like) are practical for stripping easy to remove (not baked or baked at 135.degree. C. maximum) positive photoresists. For positive photoresists baked higher than 150.degree. or exposed to plasma or ion implantation processing, it is often advantageous to use stripping solutions consisting of amines or amines diluted with neutral polar solvents.
It is desirable to operate photoresist stripping at low temperatures primarily for safety reasons minimizing both fire hazards from solvent fumes and contacting personnel with hot liquids and vapors. Lower temperature operation also minimizes problems from changing composition due to selective vaporization of more volatile components.
One of the major drawbacks in stripping solutions which are effective for vary hard to strip material is that they usually contain environmentally hazardous or toxic ingredients such as phenolics or chlorinated hydrocarbons. Accordingly, a need exits for an improved environmentally safe, relatively non-toxic positive photoresist stripping composition which is effective in removing hard to strip positive photoresists.